234 Met?iod of separating Small Quantities 



the funnel carefully, so as to surround the zinc, avoiding, as far 

 as possible, to mix it with the water below, and the stopcock and 

 its appendages are to be replaced in the mouth of the tube ; the 

 production of the gas then goes on as before stated, and the 

 mode of manipulating with it is exactly the same as described in 

 the foregoing part of this paper. 



It will be necessary for me, in this place, to explain the me- 

 thods I employ after each operation, to determine the integrity 

 of the instrument, so as to satisfy myself that no arsenic remams 

 adhering to the inside of the tube, or to the cork and its appen- 

 dages, before I employ it ibr another operation. 



After washing the apparatus with clean water, a piece of zinc 

 may be dropped in, and the tube filled to within '.alf an inch of 

 the top of the short limb ; two drachms of diluted sulphuric acid 

 are then poured in, and the stopcock and cork secured in its 

 place ; hydrogen gas will in this case, as before, be liberated, 

 and fill the tube. If the gas as it issues from the jet be then 

 inflamed, and a piece of window-glass held over it as before de- 

 scribed, and any arsenic remains, it will be rendered evident by 

 being deposited on the glass ; if so, this operation must be re- 

 peated till the glass remains perfectly clean, after having been 

 exposed to the action of the gas. 



When I have had an opportunity of working with so large a 

 quantity of mixture as from two to four pints (imperial measure), 

 I then have employed the instrument here figured, which is, in- 

 deed, but a slight modification of one of the instantaneous light 

 appearances, now so well known and used for obtaining fire by the 

 aid of a stream of hydrogen gas thrown on spongy platinum. 

 It will, therefore, be of importance only for me to describe the 

 alteration which I make when I employ it for the 

 purpose of detecting arsenic. In the first place, I 

 must observe, that the outer vessel a, which I use, 

 holds full four pints, and that the jet of the stop- 

 cock is vertical, and its orifice is twice or three times 

 larger than in the instrument as generally made 

 for sale, and also that there is a thread or wire at- 

 tached to the cork of the stopcock 5, for suspending 

 a piece of zinc c, within the bell-glass. 



With an instrument of this description, I have operated on 



